The present invention relates to a magnetic sensor for measuring the magnetic field of a measurement target with a SQUID (Superconducting Quantum Interference Device).
Conventionally, as a magnetic sensor for measuring the two-dimensional magnetic field distribution of a measurement target, one which guides a magnetic flux from the measurement target to a SQUID through a needle-like flux introducing member is known. When a needle-like flux introducing member is used in this manner, the magnetic field of the measurement target can be measured in units of small regions, so the position resolution can be improved. In the magnetic sensor using the SQUID in this manner, the SQUID must be cooled to an operating temperature. For this reason, the SQUID, needle-like flux introducing member, and measurement target are all arranged in a vacuum container in order to prevent external heat transfer by vacuum heat insulation.
The above conventional magnetic sensor has the following problems. More specifically, in order to hold the SQUID at a low temperature, the measurement target is also set in the vacuum container, as described above. Accordingly, a cumbersome operation of placing the measurement target in the vacuum container is necessary, which takes time for measurement preparation. To facilitate measurement preparation while cooling the SQUID, the measurement target may be set outside the vacuum container. This increases a distance from the measurement target to the needle-like flux introducing member, and the measurement precision of the magnetic field distribution degrades.
The present invention has been made in view of the above situation, and has as its object to provide a magnetic sensor in which a cooling efficiency for a SQUID is high and measurement preparation is easy, and which can perform magnetic field measurement at high precision.
In order to achieve the above object, according to the present invention, a magnetic sensor for measuring a magnetic field distribution of a measurement target by using a SQUID is characterized by comprising a storing portion with an interior held in a vacuum state to store the SQUID, and a rod-like flux introducing member made of a high-permeability material to guide a magnetic flux from the measurement target to the SQUID, wherein the flux introducing member has one end located in the storing portion to be away from the SQUID and the other end located outside the storing portion.
The magnetic sensor according to the present invention has the rod-like flux introducing member with one end located in the storing portion and the other end located outside the storing portion. The magnetic flux from the measurement target can be guided to the SQUID by moving that side of the flux introducing member which is located outside the storing portion to the measurement target. In this manner, according to the present invention, since the other end side of the flux introducing member is located outside the storing portion, the distance between the flux introducing member and measurement target can be decreased even if the measurement target is not set in the container. Thus, cumbersome measurement preparation is not required, and measurement precision for the magnetic field distribution can be improved. Since one end of the flux introducing member is away from the SQUID, heat from the other end side of the flux introducing member is not easily transferred to the SQUID, so the SQUID can be cooled efficiently.
Preferably, a flux absorbing member made of a high-permeability material is further provided around the flux introducing member in order to surround a portion of the flux introducing portion which is located outside the storing portion. When this arrangement is employed, the external magnetic flux from except the measurement target can be absorbed by the flux absorbing member. Thus, the measurement precision for the magnetic field distribution of the measurement target can be further improved.